More and more sensors are being used to control and regulate operations in motor vehicles. This means that more and more sensor signals must be transmitted (independently of one another) to the control and regulation units. Individual signals must, in this context, meet different requirements. If the individual signals are combined into a "summed signal," the individual requirements must still be met.
It is known to measure the rotational speeds of the vehicle wheels to regulate or control braking force, drive power, and/or driving dynamics. A number of methods (for example Hall-effect sensors or magnetoresistive sensors) are indicated for this purpose in the art. It is also known to determine the wear on the brake lining of a motor vehicle brake by, for example, embedding contact pins at a certain depth in the brake linings, with the pins connecting with a contact when the brake lining has worn to that depth.
Publication "Integrierte Hall-Effekt-Sensoren zur Positions- und Drehzahlerkennung" (Integrated Hall-effect sensors for detecting position and rotational speed), elektronik industrie 7, 1995, pp. 29-31, describes how active sensors (Hall-Effect sensors) used in motor vehicles for anti-lock braking, automatic slip control, engine and transmission control and regulating systems. In a two-wire configuration, such sensors supply two current levels which are converted into two voltage levels by a measuring resistor in a corresponding control unit.
The use of magnetoresistive sensors for rotational speed sensing is also described in the publication "Neue, alternative "Losungen fur Drehzahlsensoren im Kraftfahrzeug auf magnetoresistiver Basis" (New alternative solutions for magnetoresistively based rotational speed sensors in motor vehicles), VDI-Berichte No. 509, 1984.
German Patent No. 26 06 012 (corresponding to U.S. Pat. No. 4,076,330) describes a specific shared arrangement for sensing the wear on a brake linking and for sensing wheel rotational speed. The sensed brake lining wear signals and the wheel rotational speed signals, which are sensed using an inductively operating sensor, are sent through a shared signal line to an analysis unit. This can be achieved since the wheel rotational speed sensor is entirely or partly short-circuited in response to the sensing of brake lining wear.
Other systems described, for example, in German Patent No. 43 22 440 require at least two signal lines between a wheel unit and the analysis unit in order to detect rotational speed and brake lining wear at a wheel and a wheel brake.
With respect to the rotational speed sensing, it is known that an air gap between a rotating ring gear and actual sensor element has a considerable influence on the quality of the rotational speed signal, as described in German Patent No. 32 01 811.
Information about the rotational direction of the wheels is also required, for example, for "hill-holder" systems. Such information is required to indicate whether the vehicle is moving backward. This is described in German Patent No. 35 10 651.
The aforementioned data (e.g., brake lining wear, air gap and rotational direction), as well as other data, are generally sensed close to the wheel and analyzed in a control unit, which is remotely positioned from the wheel. The data must be transmitted to the control unit for this purpose.
In the case of a motor (internal combustion and/or electric motor), it is known to sense the motor rotational speed with inductive, magnetoresistive, or Hall-effect sensors.
In German Patent Application No. 1 96 09 062.8, it is indicated that the data of an analog rotational speed signal that periodically includes two presettable current or voltage levels. The presettable current and voltage levels have digital data regarding the brake lining wear, air gap, and/or rotational direction superimposed on them so that the current or voltage level of the analog rotational speed signal is modified in coded manner.
Increasing the current level in order to transmit the additional digital data is advantageous since only one two-wire connection between the sensor unit and the control unit is necessary. Increasing the current level results, however, not only in an increase in dissipated power but also in an increased voltage drop at the measuring resistor in the control unit. Changing the voltage level does not increase the dissipated power, but does require a three-wire connection (voltage supply, ground and signal line) between the sensor unit and the control unit.
It is the object of the present invention to superimpose digital data onto an analog signal in the simplest and most reliable manner possible.